JPS5841762B2 - Polymer electret film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electret film using a polymeric material as a dielectric.

As is well known, electrets permanently maintain electrical polarization through special polarization treatment, and have been known for a long time.

The electret phenomenon is seen in more or less insulating materials, but electrets that are excellent enough to be put to practical use have now been made from artificial synthetic polymer materials.

Polymer materials used for the electret include tetrafluoroethylene-hexafluoropropylene copolymer, polyethylene terephthalate, polyvinylidene fluoride, polymethyl methacrylate, and polypropylene.

However, despite having the potential to be used in a wide range of fields in the future, electrets are currently limited to special audio equipment because current electrets have a sufficiently high surface charge density (or surface potential). ) cannot be obtained and its stability is not sufficient.

The present inventor has found from various research results that an electret film made of a uniaxially stretched or biaxially stretched film of poly[3゜3-bis(chloromethyl)oxacyclobutane] exhibits a higher surface potential than an unstretched film. It was found that its stability was also excellent.

The reason why poly[3,3-bis(chloromethyl)oxacyclobutane] exhibits superior electret properties compared to other polymers is due to the unique properties of poly[3,3-bis(chloromethyl)oxacyclobutane]. We inferred that it was due to crystallinity, and conducted various studies, and as a result, it became clear that singular crystallization occurs due to the electric field during the electretization process.

However, we have now discovered that even if the film is made into an electret after undergoing the unique crystallization process, it exhibits similarly excellent electret properties.

The unique crystals can be easily generated by stretching.

The unique crystal of poly[3,3-bis(chloromethyl)oxacyclobutane] is usually called the β-type crystal.
Because it has a crystal structure without symmetry, the crystal itself has spontaneous polarization.

This β-type crystal is obtained by stretching at 130° C. or lower.

The stretching may be uniaxial or biaxial. In addition, the stretching speed was 130
Any speed may be used at temperatures below °C.

Furthermore, in order to increase the amount of β-type crystals, the die temperature must be 180°C.
It is obtained by extruding it into a sheet from an extruder at ~240°C, immediately passing it through cold water at 10°C or lower to obtain a sheet mainly consisting of amorphous material, and stretching this sheet at 130°C or lower.

Further, a sheet that has already been crystallized in the α-type is also transformed into the β-type by stretching at a high magnification at 100° C. or lower.

In this way, a stretched film of poly[3,3-bis(chloromethyl)oxacyclobutane], which is mainly composed of the β type, is processed by the thermal electret method - that is, the film is sandwiched between flat electrodes and a DC voltage (polarization voltage) is applied. The temperature is raised in a housing to a certain temperature (polarization temperature), held at that temperature for a certain period of time (polarization time), and then lowered and the voltage removed to form an electret.

Polarization voltage is 5 to 100V per 1μm, polarization temperature is 100V
°C to 160 °C, and the polarization time is 30 minutes or more.

The stable surface potential of the electret film thus obtained is 650 to 5 oov, which is higher than the 350 to 500 V of the unstretched film.

The reason for this is that in an unstretched film, β-type crystals are oriented and generated by the electric field, but in the case of a stretched film, a large amount of β-type crystals are generated in advance, and the electric field only acts to orient the crystals. This is because the amount of oriented β-type crystals is large.

As described above, an electret film using a stretched film of poly[3,3-bis(chloromethyl)oxacyclobutane] has a high surface potential and good stability, and therefore has high industrial value.

Next, examples of the present invention will be described.

Example 1 Poly(3,3-bis(chloromethyl)oxacyclobutane) was molded into a sheet using an extruder.

The extruder barrel die temperature was 182-232°C.

This sheet was stretched at 90°C using a simultaneous biaxial stretching machine using the tenter method.
It was stretched with

The stretching ratio was 4 times FC in the longitudinal direction and 4 times in the transverse direction.

The film was then stabilized by heat treatment at 95°C.

This film was made into an electret using a conventional thermal electret method.

The polarization conditions are a 40 μm film and a polarization voltage of 200 μm.
, the polarization temperature was 115°C, and the polarization time was 2 hours.

The surface potential of this film was 90 immediately after being made into an electret.
0■, 853V after being left at a temperature of 70 for 1 hour,
It was 813 ■ after leaving it for one day, and 795 ■ after one month.

The surface potential was measured with an electrostatic field meter.

In addition, in the case of an unstretched 40 μm film, it was 360 to 490 μm.

Example 2 A sheet of poly(3,3-bis(chloromethyl)oxacyclobutane) was uniaxially rolled and stretched at 60° C. using a roll mill to form a film with a thickness of 100 μm.

This film was made into an electret in the same manner as in Example 1 to obtain a surface potential of 830 ■.

The stretching ratio was about 5 times. Example 3 Poly[3,3-bis(chloromethyl)oxacyclobutane] was extruded into a sheet using an extruder and immediately heated to KO°C.
The sheet was then stretched 5 times at 80℃ using a batch-type small-scale simultaneous biaxial stretching machine, and stretched to 12
Heat treatment was performed at 0°C.

This film was made into an electret under the same polarization conditions as in Example 1 to obtain a surface potential of 980 ■.

As is clear from the above examples, the polymer electret film of the present invention has a higher surface potential and is more stable than conventional electret films, and has high utility value.

Claims (1)

[Claims] 1 A polymer electret film obtained by converting a poly(3,3-bis(chloromethyl)oxacyclobutane)-based uniaxially or biaxially stretched film into an electret.